(596e) First Principles Monte Carlo Simulations of Elemental Fluid Phase Equilibria

We use modern Monte Carlo sampling methods in concert with standard electronic structure codes (CPMD and NWChem) to perform liquid-vapor and liquid-liquid coexistence calculations from first principles. In order to improve computational efficiency an approximate "pre-sampling" potential is used to generate large moves with a high probability of acceptance, though the actual distribution of sampled states is determined only by the first-principles energy calculations [1].

The liquid elements, especially in the P-block, display a variety of liquid-liquid transitions (most famously in phosphorous [2] and sulfur [3], but also in selenium and others [3]) in addition to normal vaporization equilibria, as well as unusual liquid structure in many cases. All of this behavor is due to the strongly-directional and many-body covalent nature of interactions in these systems. While there has been much recent progress in developing empirical potentials for such chemically reactive systems [4], the use of "potential-less" first-principles methods is particularly appealing in studies of these systems; the relatively high temperatures considered also make it more likely that the known deficiencies in modern density functional methods (poor or inconsistent treatment of dispersion forces, etc.) are less likely to substantially affect the results.

Issues of efficiency, correctness and load-balancing arising from the use of iterative density functional theory potentials are addressed both directly and through analysis of model systems. System size effects are also tested using model systems. A hybrid approach in which two "levels" of pre-sampling is constructed, in which the first uses an empirical potential, and the second uses density functional theory but with a very low (and fixed) number of plane waves. Construction of appropriate pseudopotentials and choice of exchange-correlation functional are evaluated in detail for both metallic and insulating systems.

[1] Gelb and Carnahan, Chem. Phys. Letts. 4-6 (2006) 283.

[2] Katayama et al., Nature 403 (2000) 170.

[3] Brazhkin et al., Physica B 265 (1999) 64.

[4] van Duin et al., J. Phys. Chem. A 105 (2001) 9396.